1. Field of the Invention
The present invention relates to an optical beam scanning system comprising a deflector for deflecting a light beam from a light source and thereby scanning a surface-to-be-scanned by the deflected light beam.
2. Description of the Prior Art
FIG. 1 is a perspective view of an optical beam scanning system of the prior art. The optical beam scanning system comprises a light source 1, an optical beam modulator 1a (such as an acousto-optical modulator (AOM)), a first image-forming system L.sub.10, a deflector 4, a second image-forming system L.sub.2 and a recording drum 7. The image surface 8 of the recording drum 7 is to be scanned by a light beam B.sub.2 ; a direction X on the surface 8 (perpendicular to the direction of rotation of the drum 7) is a main scanning direction and a circumferential direction Y is a subscanning direction.
The light source 1 emits a light beam B.sub.1 of parallel rays. The light beam B.sub.1 is subjected to ON/OFF control by the optical beam modulator 1a.
The light beam B.sub.1 then passes through the first imaging-forming system L.sub.10. The system L.sub.10 consists of a beam expander 21 and a cylindrical lens 22. The beam expander 21 consists of two lenses A light beam B.sub.1a parallel rays is formed by the beam expander 21. Then, the light beam B.sub.1a passes through the cylindrical lens 22. The cylindrical lens 22 has a refracting power only along a vertical axis (equivalent to the subscanning direction Y). The focal length of the cylindrical lens 22 and a deflection as the distance surface 4a of the deflector 4. Consequently, the light beam B.sub.1a is focused on the deflection surface 4a along the vertical axis, but not along the horizontal axis.
The light beam is deflected by the deflector 4 and focused on the image surface 8 by the second image-forming system L.sub.2. The deflector 4 is a rotary polygon mirror; the polygon mirror 4 has several mirror surfaces (or deflection surfaces) 4a. The mirror 4 rotates to deflect the light beam in the main scanning direction X. The second image-forming system L.sub.2 consists of an f.theta. lens 5 and a cylindrical lens 6 having a refracting power along the vertical axis. The reflection surface 4a and the image surface 8 are at conjugate positions along the vertical axis of the image-forming system L.sub.2.
The resolving power of the prior art optical beam scanning system is inversely proportional to the cross-sectional size of the light beam B.sub.1a. That is, the resolving power of the overall system is increased by expanding the initial light beam B.sub.1 to the light beam B.sub.1a.
Further, the second image-forming system L.sub.2 prevents scanning pitch irregularity in the subscanning direction Y. Such irregularity can be caused by a facet error such as inclination of the deflection surface 4a.
However, the prior art system has some disadvantages. The provision of the two lenses of the beam expander 21 increases the overall length of the optical path and increases the number of optical elements. This increases the cost of the first image-forming system L.sub.10 and makes installation and arrangement of its optical elements more laborious.